70V audio wiring size

[retirede]

Good design practice dictates that you choose an amp with at least 10% more power than you need. But under no circumstances should you exceed the rated power output.



Happens all the time. You'll also find regular 8 ohm speakers hung on 70 volt lines which will really drop the impedance. Probably won't cut out since amps are pretty well protected but you won't get the volume you expect.

A non-recommended way to find those illegal speakers without transformers is to disconnect the wiring from the amp and apply 120 volts. Those 8 ohm speaker voice coils will vaporize.

-Hal

On large installations, only the speaker without transformer will produce much sound. The low impedance will consume almost all available power.

I was called to help with a school that the custodian/handyman was having trouble with such a system.

I asked what was going on and he told me that he added a new speaker and it was the only one working. Once he realized that the only one that worked had no transformer, he rewired all the others to bypass the transformer.

 

[ChukGleason]

On large installations, only the speaker without transformer will produce much sound. The low impedance will consume almost all available power.

I was called to help with a school that the custodian/handyman was having trouble with such a system.

I asked what was going on and he told me that he added a new speaker and it was the only one working. Once he realized that the only one that worked had no transformer, he rewired all the others to bypass the transformer.

I used to work for a muni radio shop, and we were redoing the fire station trip-lights and dispatch speakers in over 25 stations. 70V distrib, locked cabinet for the radio & PA (idle firefighters fingers are ahhhhh, chaotic)

Got a call from one I had done about 6 months before. "The speaker on the back patio is too loud, can you come turn it down?"

I didn't remember putting an outside speaker at that station but I often drop bits...

Found: Radio Shack 8 ohm scanner speaker, mounted over rear door, zip-cord run inside to the closest 70V speaker, and yes, right across the line. Points for good wire-ties along the way.

Negatives: 1) Station personnel weren't supposed to touch the system (I mean, locked cabinet, ya know) 2) Dept Chief had already long ago decreed no speakers on back patios (being nice neighbors to people behind, and anytime outside the station FF's supposed to carry their trunked radio.

I asked FF helping me, who did this. "I did" Who told you to do this? "My captain"

Snip-snip. And report to FD management. Somebody probaby got reprimanded.

 

[WasGSOHM]

There is a thing called Damping Factor and it has to do with the impedance seen looking into the amps output.

Less impedance is better and I've seen speaker cables that were woven ribbons for very low inductance per foot.

This cable might have been snake oil, though.

 

[GoldDigger]

There is a thing called Damping Factor and it has to do with the impedance seen looking into the amps output.

Less impedance is better and I've seen speaker cables that were woven ribbons for very low inductance per foot.

This cable might have been snake oil, though.

Good damping factor (low source impedance compared to load impedance) is most important with large speaker cone excursions (which come from low frequency drive). And the effect is greatest at frequencies near the mechanical/acoustic resonant frequency of the driver/enclosure combination. The inductive impedance of the cable is lowest (negligible) at those low audio frequencies. Based on that, I call snake oil on most if not all of those claims.

The source impedance of the amplifier, on the other hand, is likely to be fairly constant across the frequency range.
If the resistive impedance of the wire run is more than 10% of the load impedance, then in addition to losing power you are likely to be able to measure (but not necessarily notice) increased distortion at low frequencies and greater variation from flat in the frequency response curve of the system.

 

[GoldDigger]

Once he realized that the only one that worked had no transformer, he rewired all the others to bypass the transformer.

The equivalent of turning the volume control up to 11?

 

[James L]

There is a thing called Damping Factor and it has to do with the impedance seen looking into the amps output.

Less impedance is better and I've seen speaker cables that were woven ribbons for very low inductance per foot.

This cable might have been snake oil, though.

It's not snake oil, but it's not for your average, run-of-the mill system, either.

Most people would never hear the difference, and most systems aren't good enough to warrant it

 

[LarryFine]

Damping factor reflects the amp's ability to stop speaker-cone movement when the signal stops, as well as getting it moving when the signal starts. It's all about keeping the speaker movement under control, so it reflects the desired output fidelity.

In my home theater system, I use a fine-stranded 12-gauge speaker wire. My main speakers are bi-wired, with the high- and low-frequency drivers being driven from separate speaker terminals. (I have Sunfire Stereo and Cinema Grand amps)

 

[synchro]

In my home theater system, I use a fine-stranded 12-gauge speaker wire. My main speakers are bi-wired, with the high- and low-frequency drivers being driven from separate speaker terminals. (I have Sunfire Stereo and Cinema Grand amps)

Do you have an active crossover separating the high and low frequencies before driving the amps?

 

[LarryFine]

Do you have an active crossover separating the high and low frequencies before driving the amps?

No, but the outputs of the Stereo and the left-front and right-front outputs of the Cinema Grand have what are called Voltage Source and Current Source terminals; the latter have 1-ohm resistors in line with the + terminal.

The Voltage Source terminals are direct outputs for low frequency or full-range speakers (only one terminal pair), and the Current Source outputs are supposed to resemble the "softer" sount of tube amps, for high frequencies.